CH 54 POPULATION ECOLOGY
1. Understanding Populations
3. How populations grow
4. Human population growth
A population can be defined as a group of interbreeding individuals occupying the same area at the same time.
Population ecology is the study of how populations grow and what factors promote and limit growth.
To study populations, we need to use demography, the study of birth rates, death rates, age distributions and the sizes of
54.1 Understanding Populations
- Population density: the numbers of organisms in a given unit area.
Ecologists use many different methods to quantify population density
- Simplest way to measure population density is to visually count number of organisms in a given area and this works
only if area is small and organisms are relatively large.
o Use suction traps, pitfall traps, mist nets, snap traps can catch animals to find number of animals caught per
unit area per habitat.
- Mark-recapture technique: capture animals and then tag and release them.
o We assume that the ratio of marked individuals is the same as the ratio of unmarked individuals.
o Drawback is some animals that have been marked may learn to avoid the traps and recapture rates will then
be low, resulting in an overestimate of population size. Or animals are trap-happy, especially if the traps are
baited with food.
- We can also count pelts, catch per unit in fishery, pellet numbers, chorusing, or ground covered by plants.
Patterns of Spacing
- Individuals within a population can show different patterns of spatial dispersion, they can be clustered together or
spread out to varying degrees.
o Can be clumped, uniform, or random.
- Clumped: most common dispersion pattern likely because resources tend to be clustered in nature
- Uniform: competition may cause this dispersion pattern.
o As between trees in a forest, where competition between roots may cause some trees to be outcompeted by
others, causing a thinning out uniform distribution.
o May also result from social interactions, as between some nesting birds, which tend to keep an even distance
from each other.
- Random: rarest dispersion pattern because resources in nature are rarely randomly spaced.
o Dispersion patterns of plants may be random and lacking a pattern.
- Semelparity: producing all offspring in a single reproductive event. This is common in insects and other invertebrates
and organisms such as salmon, bamboo grasses, and agave plants.
- Iteroparity: reproducing several times throughout life cycle.
o Ex. Most vertebrates and perennial plants such as trees.
Age Classes - The reproductive strategy employed by an organism has a strong effect on the subsequent age classes of population.
- Cohorts: same-aged young that are produced by semelparous organisms.
- A population increasing in size should have a large number of young, whereas a decreasing population should have few
- Imbalance in age classes can have a profound influence on a population’s future.
o Bigger, older reproductive age classes are often removed.
o Removal of younger age classes are also experienced.
- A life table provides data on the number of individuals alive in each particular age class.
- Demography: study of how births and deaths change population sizes over time.
Life Tables and Survival curves summarize survival patterns
- Laws that protect beavers allowed their population to recover, often growing to nuisance status.
- Trapping was supported as a management technique and beaver teeth were used for age classification.
- D x n –xn x+1
- The number dying in a given age class or year = number alive at start of previous year - number alive at start of next
- Survivorship curve: Value of n , nxmber of individuals is expressed on a log scale, and log scales are used to
examine rates of change with time.
- Survivorship curves fall into one of 3 types:
1. Type 1 curve: rate of loss for juveniles is relatively low and most individuals are lost later in life as they become
older and more prone to sickness and predators.
o Ex. Large mammals, humans
2. Type 2 curve: middle ground, with fairly uniform death rates over time.
o Ex. Birds, small mammals, reptiles, annual plants.
3. Type 3 curve: rate of loss for juveniles is relatively high and survivorship curve flattens out for those organisms
that have survived early death.
o Fish and marine invertebrates.
Age-Specific Fertility data can tell us when to expect growth to occur
- Age-specific fertility rate: (m x data from determining the proportion of female offspring that are born to females of
- Net reproductive rate = sum of age specific survivorship x age-specific fertility
R 0 ∑l mx x
- To calculate the future size of a population, we multiply the number of individuals in the population by the net
o N = NR
t+1 t 0
- If R0 > 1 population will grow and vice versa for R0<1. If R0 = 1, then population is at equilibrium.
54.3 How Populations Grow
Per Capita Growth Rate
- R = B-D (rate = births per unit time – deaths per unit time)